1. Field of the Invention
The present invention relates to photosensing semiconductor apparatus and particularly to photosensing semiconductor apparatus which measure light using photodiodes and operational amplifiers.
2. Related Background Art
One conventional photosensing semiconductor apparatus will be described with respect to the drawings.
FIG. 1 is a schematic of a conventional photo-sensing semiconductor apparatus. FIG. 2 is a circuit diagram of the first stage of an operational amplifier used in the apparatus.
In FIG. 1, reference numeral 14 denotes a photodiode; reference numeral 12, a nonlinear element which includes, herein, a logarithmic compression diode; and reference numeral 13, an operational amplifier. Diode 12 is connected between the inverting input (-) and output terminals of amplifier 13. Photodiode 14 is connected between the inverting input and non-inverting input terminals of the amplifier.
In FIG. 2, references 1, 2 denote active elements which includes, herein, MOSFETs, which constitute a differential input. Reference numerals 3, 4 denote active elements, herein NPN transistors, which constitute active loads for transistors 1, 2. Reference numerals 5, 6 denote offset reducing resistors, the resistances of which are equal. Reference numeral 7 denotes a constant-current source which supplies a biasing current. Reference numeral 8 denotes an offset adjusting resistor connected between external terminals 9 and 10.
The trouble with the operational amplifier of the conventional photosensing semiconductor apparatus is that the offset adjusting resistor must be provided and adjusted so as to nullify the offset voltage of the amplifier due to unavoidable nonuniform manufacture of operational amplifiers.